System and method of determining cardiac pressure

ABSTRACT

A pressure sensor is deployed in the right atrium and is in contact with the tissue of the fossa ovalis. The fossa ovalis acts as a membrane and the pressure sensor determines the relative and/or absolute pressure within the left atrium while remaining within the right atrium. A variety of embodiment are provided to deploy and anchor the sensor into the proper position.

FIELD OF THE INVENTION

The present invention relates to implantable medical devices. Morespecifically, the present invention relates to implantable medicaldevices that sense or measure pressure.

DESCRIPTION OF THE RELATED ART

There are a number of implantable medical devices (IMDs) that sensevarious physiological parameters and/or provide a variety of therapies.For example, implantable pulse generators (IPG) typically include one ormore leads that are in contact with cardiac tissue to sense electricaldepolarization and provide pacing stimuli. Implantablecardioverter/defibrillators (ICD) also typically include one or moreleads and provide a larger stimulus for cardioversion or to defibrillatethe heart. Often, IMDs include both pacing andcardioversion/defibrillation capabilities.

A housing containing the pulse generator, battery, capacitors,processor, memory, circuitry, etc. is implanted subcutaneously. One ormore leads are delivered transvenously such that electrodes forming aportion of the lead are disposed within or contacting an outer portionof the heart. The housing, or “can”, may also include one or moreelectrodes that are selectively used in combination with the variouslead electrodes.

In general, the leads sense electrical activity of the heart, typicallyrepresented as an electrogram (EGM), which is indicative of the cardiacdepolarization waveform and indicates the timing of the variouscomponents of the complex. This data indicates whether and whenintrinsic events occur, their duration and morphology. The timing ofcertain events (or their failure to occur when expected) is used totrigger various device actions. For example, sensing an atrialdepolarization may begin a timer (an escape interval) that leads to aventricular pacing pulse upon expiration. In this manner, theventricular pacing pulse is coordinated with respect to the atrialevent.

The heart includes four chambers; specifically a right and a left atriumand a right and left ventricle. Leads are commonly and routinely placedinto the right atrium as well as the right ventricle. For left sidedapplications, the lead is typical guided through the coronary sinus andinto a cardiac vein. One or more electrodes are then positioned (withinthe vein) to contact an outer wall of the left atrium and/or leftventricle. While direct access to the interior of the left atrium andleft ventricle is possible, it is generally less preferable. As the leftventricle provides oxygenated blood throughout the body, any foreignobject disposed on the left side could lead to the formation of clotsand would increase the risk that such a clot would form and bedispersed.

The sensing and utilization of electrical data is commonly employed asthe electrodes used for delivering stimulus are typically also useful insensing this data. This is generally non-problematic in left-sidedapplications as the electrical waveform is adequately sensed from theabove described left side lead placement position.

A wide variety of other sensors are employed to sense parameters in andaround the heart. For example, flow rates, oxygenation, temperature andpressure are examples of parameters that provide useful data in certainapplications. Obtaining such data on the right side is typicallynon-problematic; however, obtaining the same data directly from the leftside is made more difficult by the general inability (or undesirability)to place a sensor or component into the left atrium or ventricle.

Pressure data, in particular, is a useful parameter in determining thepresence, status and progression of heart failure. Heart failure oftenleads to an enlargement of the heart, disproportionately affecting theleft side. Left side pressure values would be useful in monitoring thepatient's condition; gauging the effectiveness of a given therapy suchas Cardiac Resynchronization Therapy (CRT); and timing, controlling ormodifying various therapies. Of course, the direct measurement of leftsided pressure values is made difficult because pressure sensorsgenerally are not implanted within the left atrium or left ventricle.

Left atrial pressure, in particular, is a variable that defines thestatus of heart failure in a patient. Attempts have been made to measuresurrogates of this variable by monitoring pulmonary wedge pressure inclinical care. Measurement of ePAD with implantable devices such as theMedtronic Chronicle™ have been used to measure real-time intracardiacchamber pressure in the right ventricle and provide an estimate of meanleft sided pressure. These techniques generally do not provide certainphasic information and do not necessarily correlate with left atrialpressures under certain conditions such as pulmonary hypertension orintense levels of exercise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an implantable medical device (IMD) having aplurality of leads implanted within a heart.

FIG. 2 is a block diagram illustrating the functional components of anIMD.

FIG. 3 is an illustration of a heart showing an interior view of a rightatrium and indicating the location of the fossa ovalis.

FIGS. 4A-4D are schematic diagrams illustrating an embodiment of apressure sensor assembly having deployable anchor prongs.

FIGS. 5A-5E are schematic diagrams illustrating an embodiment of apressure sensor assembly having rotatably deployable anchor prongs.

FIGS. 6A-6B illustrate embodiment of anchor prongs.

FIGS. 7A-7D are schematic diagrams illustrating an embodiment of apressure sensor assembly having prongs that pierce through the fossaovalis.

FIGS. 8B-8C illustrate embodiments of a pressure sensor assembly thatincludes an adjustment mechanism the moves the pressure sensing capsulewithin the housing.

FIG. 9 is a flowchart of a process for providing a pressure sensor inthe right atrium to sense pressure within the left atrium.

DETAILED DESCRIPTION

FIG. 1 illustrates an implantable medical device (IMD) 10 that includespacing, cardioversion and defibrillation capabilities. A header block 12forms a portion of the IMD 10 and three leads 14, 16, 18 are illustratedas coupled with the header block. A right ventricular lead 14 isdisposed in the right ventricle of the heart 20. More specifically, ahelical electrode tip 24 is embedded into the apex of the rightventricle. The electrode tip 24 forms or is part of a tip electrode anda coil electrode 26 is also included. A ring electrode may be disposedbetween the tip electrode 24 and the coil electrode 26.

An atrial lead 16 is disposed within the right atrium such than anelectrode 28 contacts an interior wall of the right atrium. A left sidedlead 18 is illustrated as passing through the coronary sinus 22 and intoa cardiac vein. In this position, the left sided lead 18 has a distalend in contact with an outer wall of the left ventricle. The IMD 10includes a housing that can act as an electrode or, though notillustrated, may include multiple electrodes. With such a configurationpacing stimuli is selectively delivered to the right atrium, the rightventricle, and/or the left ventricle. Likewise, a defibrillation pulsemay be generated from any given electrode to any second electrode, suchthat the defibrillation waveform traverses the desired portion of theheart 20.

FIG. 2 is a simplified schematic diagram illustrating certain componentsof the IMD 10. The IMD 10 includes a processor or CPU 1306, memory 1310,timing circuits 1314, timing output circuit 1304, pacing anddefibrillation output circuits 1302, an appropriate lead interface 300,and appropriate electrode sensing circuits 1316. The operation of theIMD 10 may be controlled by software or firmware and may be reprogrammedand/or provide data to an external device via telemetry unit 1318.

Also illustrated are exemplary sensing units that may be included withIMD 10. For example, an activity sensing circuit 322, and a minuteventilation circuit 1308 are included. Thus far, IMD 10 is illustratedin an exemplary manner and may or may not include all componentsillustrated and may include many additional components and capabilitieswithout departing from the spirit and scope of the present invention.

A pressure sensing circuit 1312 receives input from the pressure sensordescribed herein. In one embodiment, a pressure sensor is included onthe right atrial lead 16. The pressure data, when received, is used bythe CPU 1306 to monitor or control therapy, monitor the status of theheart, and/or to provide information to an external device via telemetryunit 1318. It should also be appreciated that various pressure sensorsmay provide relative data and an absolute pressure sensor (not shown)may be positioned external to the heart and utilized to providereference data via telemetry unit 18 and/or to the external device.

FIG. 3 is an illustration of the anatomy of a human heart 20. Inparticular, the interior of right atrium 30 is illustrated, along withthe superior vena cava 32 and inferior vena cava 34. The atrial septum,dividing the right atrium from the left atrium is primarily defined(from the right side perspective, by the fossa ovalis 36. Surroundingthe fossa ovalis 36 is the fossa limbus 38, which is a raised muscularrim. The fossa ovalis 36 is a relatively thin, but very strong membranethat separates the right atrium from the left atrium and is anon-conductive pathway for depolarization. The fossa ovalis 36 marks theprevious location of the foramen ovale, which in embryonic and fetaldevelopment provided for direct passage between the atrial chambers. Thefossa limbus 38 and the atrial tissue surrounding the fossa limbus 38 isconductive.

FIG. 4A is a schematic illustration of a pressure sensor assembly 50coupled with the atrial lead 16′. The pressure sensor assembly 50 isdisposed within the right atrium 30 and its position relative to theleft atrium 40 is illustrated. More specifically, the pressure sensorassembly 50 is in contact with the fossa ovalis 36 and in thisembodiment, held in place through the use of deployable anchoring prongs52. The anchoring prongs 52 may also serve as electrodes to pace and/orsense within the right atrium 30. In this manner, the pressure sensorassembly 50 utilizes the fossa ovalis 36 as a portion of a pressuresensing configuration to measure the left atrial pressure from withinthe right atrium 30. That is, by controlling for the effects of theright atrial pressure, the membrane of the fossa ovalis 36 (or a portionthereof) will deflect proportionally to the fluid pressure exerted inthe left atrium 40 and provide direct, real time pressure indications.These pressure indications will provide relative pressure values as wellas pressure changes (deltas) and dynamic waveform morphologies. With theinclusion of an external pressure reference sensor, such values couldalso be correlated to absolute pressure values.

FIG. 4B is a schematic side sectional view of the pressure sensorassembly 50. The assembly 50 includes a housing 70 that is coupled withthe lead 16′. Disposed within the housing 70 are one or more anchorprong tracks 55, with track 55 a, 55 b illustrated. In this embodiment,there are a total of four such tracks each having a corresponding anchorprong 52, with prongs 52 a and 52 b illustrated. Coupled or contacting aproximal portion of each anchor prong 52 is a prong deployment mechanism58. The prong deployment mechanism 58 deploys or retracts the anchorprong 52 along the anchor prong tract 55 from the proximal end of thelead 16′. Because the housing 70 contacts the fossa ovalis 36, theanchor prong tracts 55 direct the anchor prongs into the tissue of thefossa limbus 38. As the shape of the fossa ovalis 38 and correspondinglythe fossa limbus 38 will vary from patient to patient, the distance fromany given prong tract 55 to limbus tissue may vary. Thus, the anchorprongs 52, in one embodiment, may be independently advanced in varyingdistances to account for this anatomical variation. Alternatively, theprongs 52 are each made sufficiently long to accommodate wide variationsin distance. This may result in a given prong 52 piercing through thelimbus 38 and into or along the surrounding atrial tissue. This isnon-problematic and provides an even greater area of contact between theprong 52 and conductive tissue. As such, independently advanced prongs52 should be advanced at least a minimal distance into the limbus toassure anchoring and further advancement is optional, but potentiallybeneficial both from an anchoring perspective as well as forpacing/sensing capabilities.

The prong deployment mechanism 58 is a relatively stiff member that isdirectly advanced or retracted to effect deployment or retraction of theprongs 52. Alternatively, the prong deployment mechanism could include athreaded portion such that rotation of the deployment mechanism 58effects lateral movement and a corresponding advancement or retractionof the prongs 52. In an alternative embodiment, the prong deploymentmechanism 58 may be selectively decoupled from the prongs 52.

Referring to FIGS. 6A and 6B, two prong embodiments are illustrated. Itshould be appreciated that the prong embodiments illustrated, thosedescribed in the present disclosure and variations thereof areapplicable (in any combination) to any of the assembly 50 embodimentsdisclosed herein. FIG. 6A illustrates prong 200, which may be used, forexample, as prong 52 in FIGS. 5A-5D. Prong 200 includes a distalpiercing tip 210 as well as a plurality of fixed tines 205 a-205 c.Thus, forward advancement of prong 52 is permitted, but retraction isresisted by the fixed tines 205 a-205 c as they embed themselves withintissue and also serve as locations where tissue encapsulation can occur.FIG. 6B illustrates an alternative prong 200′. Prong 200′ also includesa distal piercing tip 235. Two pivoting tines 220 a and 220 b areillustrated as being pivotable about pivot point 230 a and 230 brespectively. With the tine 220 b in the retracted position, forwardadvancement is facilitated. Withdrawal of the prong 200′ (e.g., pullingbackwards during implantation) will cause the tines 220 to deploy to theextended position as tine 220 a is illustrated. While not separatelyshown, a push rod, guidewire or similar device could be coupled with astop block 245 of the tine 220 so that it may be moved from the extendedposition to the retracted position from the proximal end of lead 16′. Inthis manner, the prong 200′ could be removed or repositioned after aninitial implant. Tine 220 b is illustrated as partially protruding inthe retracted position to facilitate deployment and it should beappreciated that the tine 220 b could be fully retracted within prong200′ for removal, withdrawal or repositioning.

Prong 200′ also schematically illustrates a first electrode 240 disposedalong the main shaft. Piercing tip 235 is also indicated to be anelectrode. Multiple electrodes may be separately disposed along prong200′ or the entire prong 200′ may serve as a single electrode.

Referring to FIGS. 4A-4D, a pressure sensor 60 is centrally disposedwithin the housing 70 and is communicatively coupled with the IMD 10 viacommunication line 56. Of course, various communication protocols,including wireless protocols may be utilized. In such an embodiment,communication line 56 would represent an antenna structure. Intracardiacpressure sensing may be accomplished in a number of ways. The followingUS Patents disclose a variety of pressure sensors and are hereinincorporated by reference in their entireties: U.S. Pat. Nos. 6,223,081;6,221,024; 6,171,252; 6,152,885; 5,919,221; 5,843,135; 5,368,040;5,353,800; and 4,967,755. In the illustrated example, pressure sensor 60includes a high fidelity pressure transducer mounted on a distal end ofa capsule and in direct contact with the fossa ovalis 36, upon implant.

Phasic information of the left atrial pressure provided by the pressuresensor 60 can be used, for example, by the IMD 10 to control severalpacing parameters such as AV timing and VV timing for management of AFand CHF by optimizing left sided filling and ejection cycles and enhancecardiovascular hemodynamic performance. Such data may also be used forassessment of mitral regurgitation and stenosis. For device basedmanagement of atrial fibrillation, the phasic information can be usedfor discriminating atrial fibrillation from flutter and optimizingatrial anti-tachycardia pacing therapies.

Implantable pressure sensor 60 provides diagnostic data to cliniciansand/or control device operation by automated feedback control. Direct,real-time left atrial pressure measurement may be utilized to providediagnostic information for management of heart failure and in patientswith pacemakers, to optimize pacing parameters to prevent itsprogression. In addition, pressure sensor 60 provides information aboutthe atrial substrate for management of AF and may control pacingparameters to prevent progression of AF. Reference is made to U.S.patent application Ser. No. 11/097,408, filed on Mar. 31, 2005 andtitled “System and Method for Controlling Implantable Medical DeviceParameters in Response to Atrial Pressure Attributes,” which is hereinincorporated by reference in its entirety.

With reference to FIGS. 4B and 4D, an abrasive ring 64 is concentricallydisposed about an outer perimeter section of the distal face of thehousing 70. The abrasive ring 64 is in contact with ovalis tissue whenimplanted and stimulates the growth of endothelial or fibrotic structureto further secure the housing 70 to the specified cardiac anatomy. Inother words, the ring 64 irritates or otherwise promotes tissue growththat captures and secures at least a portion of the ring 64.

With reference to FIGS. 4B, 4C, and 4D a vacuum channel 54 is providedwithin the lead 16′ and housing 70 that terminates at a vacuum channelgrid 62. Disposed within the housing 70 and along the vacuum channel 54is a check valve 68. During implantation, the housing 70 is maneuveredinto position; this generally means that the pressure sensor 60 and/orthe housing 70 is placed into contact with the center of the fossaovalis 36. The housing 70 is firmly held against the fossa ovalis 36.Negative pressure is generated within the vacuum channel 54, for exampleby drawing a syringe coupled with a proximal access to the vacuumchannel 54 disposed near a proximal end of lead 16′. As the negativepressure is generated, the tissue of the fossa ovalis 36 is drawnagainst the vacuum channel grid 62.

The source of negative pressure may be released and the check valve 68will close and maintain the vacuum generated. This is optional, as thesource of negative pressure may be maintained throughout implant. Oncethe housing 70 is secured against the fossa ovalis 36, the anchor prongs52 are deployed and pierce the fossa limbus 38; anchoring the assembly50 into position. Over time, tissue growth in and around the abrasivering 64, the vacuum channel grid 62 and the prongs 52 further secure theplacement of the sensor assembly 50. Various external techniques may beutilized to determine that proper implantation has occurred such asfluoroscopy, X-ray, CAT scan, MRI or the like. In addition, the dataobtained from the prong electrodes 52 and/or the pressure sensor 60 canbe used to determine if proper implantation has been achieved.

Once implanted, the prongs 52 may be used as pace/sense electrodes orsimply relied upon for anchoring. The pressure sensor 60 willimmediately be able to provide data; however, until the above mentionedtissue growth occurs as well as any encapsulation about the pressuresensor 60 itself occurs, the data will change (relatively) over time.That is, the fossa ovalis 36 is acting as a transducing membrane and theresulting signal output will attenuate as this membrane changes indimension. Once stabilized, the pressure data will be most accurate. Thetiming of this tissue growth is patient dependant and may take a fewdays to a few weeks to complete. Of course, when such variation isaccounted for, the pressure data may still provide useful data evenduring this period of time.

The vacuum channel 56 and/or the check valve 68 are optional and may beleft out of various embodiments. That is, the housing 70 may bemaintained in position against the fossa ovalis 36 by various othermeans, including manipulation of the lead 16′ so that the prongs 52 aredeployable. In addition, four anchor prongs 52 and corresponding anchorprong tracts 55 have been illustrated. More or fewer may be utilized.The anchor prong 52 itself is schematically illustrated as being alinear member, but may include fixed or expandable barbs, hooks, otherattachment members and may be deformable from the linear configuration.

As indicated, the availability of, as well as the choice to use,negative pressure to secure the assembly 70 during implantation is anoption for various embodiments. Likewise, the presence of a check valve68 is another optional feature. When present and utilized, the checkvalve 68 will close and maintain negative pressure within the vacuumchannel 54 distal to the check valve 68. Over time, pressure within thisarea will increase and stabilize. The time span for this vacuumdissipation will depend upon the magnitude of the initial vacuumgenerated as well as the effectiveness of the seal naturally formedbetween the housing 70 and the fossa ovalis 36. To the extent this leadsto a slower dissipation, the vacuum effect will further anchor thedevice during the period of tissue and fibrotic growth.

In an alternative embodiment, the pressure sensor 60 is separable fromthe housing 70. The housing 70 is implanted as described either with orwithout a pressure sensor 60 in place. The pressure sensor 60 isadvanced within the lead 16′ and secured in its targeted position. Thisalternative would permit the replacement of the pressure sensor 60without requiring the removal of the housing 70. As illustrated in laterembodiments, the sensor 60 is moveable with a threaded member andcorresponding tract. Such a feature may be modified to permit thepressure sensor 60 to be completely separable from the housing 70, asdescribed.

As previously discussed, the fossa ovalis 36 will vary in size andactual shape from one patient to another. Accordingly, flexibility inthe deployment distance of the anchor prongs 52 may be provided and/orthe anchor prong length is selected to accommodate longer spans, withany excess being beneficial. Alternatively, the sensor assembly 70 maybe manufactured in a variety of standard sizes and/or shapes (e.g.,circular, elliptical, etc.). The patient's actual fossa ovalis 36 isevaluated and the most appropriate size and/or shape of the standardizedsensor assemblies 70 is chosen. Finally, custom sensor assemblies 70 maybe made based upon a specific patient's anatomical parameters.

FIGS. 5A to 5D illustrate an alternative embodiment, wherein likenumerals are used to denote similar structure to that previouslydescribed. In this embodiment, a prong support structure 105 isrotatably coupled within the housing 70. At least one, but preferably aplurality of anchor prongs 100 are coupled with the prong supportstructure. The anchor prongs 100 are positioned at a non-orthogonalangle to the prong support 105. The particular angle chosen may vary;however, the various anchor prongs 100 should be directionally aligned.That is, they should all angle in the same direction when viewed in alike manner from the frame of reference defined by the central point ofthe sensor 60.

In use, the sensor assembly 70 is positioned against the fossa ovalis36. Negative pressure may be utilized to secure the assembly into place.A prong support rotation mechanism 110 is coupled to the prong support105. The prong support rotation mechanism 110 is rotated in thedirection of arrow 120. This action causes the prong support 105 torotate and likewise cause the anchor prongs 100 to rotate. Due to theangle the anchor prongs 100 are positioned at, this rotation causes theanchor prongs 100 to pierce and enter the tissue of the fossa ovalis 36,pulling the housing 70 towards the tissue as rotation continues. Thus, adifferent angular orientation of anchor prongs 100 may result in theadvancing rotational direction to be the opposite of that illustrated.As most clearly illustrated in FIG. 5B, this angular piercing securesthe sensor assembly 70 against the fossa ovalis 36. Again, over timetissue growth and encapsulation occurs further securing the assembly 70.Though not shown in detail, appropriate slots are provided within theabrasive ring 64 to permit the travel of the prongs 100. Alternatively,the abrasive ring 64 is absent in relevant section or is constructed ofsuch a material as to permit the travel of the prongs 100.

When implanted as illustrated, the anchor prongs 100 preclude separationof the assembly 70 from the fossa ovalis 36. The more acute the angle ofthe prongs 100 with respect to the major plane of the fossa ovalis 36,the more secure the attachment will be, within reason. If the prongsupport rotation mechanism 110 were rotated in a direction opposite thatindicated by arrow 120, then the anchor prongs 100 would disengage thefossa ovalis 36. Absent sufficient tissue growth and/or the presence ofa sufficient vacuum (or other deliberate means), the sensor assembly 70would separate from the fossa ovalis 36. This is advantageous duringimplantation in that the sensor assembly 70 may be repositioned withrelative ease.

Once implantation is complete, such separation is undesirable andunintentional reverse rotation of prong support rotation mechanism 110is precluded. In order to prevent such reverse rotation, the presentinvention provides for numerous anti-reverse rotation mechanisms thatmay be used alone or in any combination. The anti-reverse rotationmechanism, in general, precludes or hinders reverse rotation to asufficient degree so that the sensor assembly 50 is reasonably andreliably secured to the fossa ovalis 36 by the anchor prongs 100 alone.In one embodiment, anti-reverse rotation mechanism is disposed in theproximal end (not illustrated) of the prong support rotation mechanism110 and includes a locking mechanism that is selectively engaged to fixthe prong support rotation mechanism 110 relative to the lead body 16′.Another anti-reverse rotation mechanism would include one or more of theanchor prongs 100 having a barb, hook, or other anchoring feature thatprecludes or hinders withdrawal of the anchor prong 100 from the tissueof the fossa ovalis 36 (e.g. FIGS. 6 a-6 b). Though not illustrated, oneor more anchor prongs 52 (FIG. 4 b) may be used in addition to theangular anchor prongs 100 to provide flexibility during implantation.That is, the rotational movement causes anchor prongs 100 to engage andwhen satisfied with placement, an anchor prong 52 including a barb orhook would be deployed to prevent reverse rotation (and to serve as anelectrode, if desired).

Another anti-rotation mechanism is a friction lock. That is, thetolerances between the prong support 105 and one or more portions of theassembly 70 are such that frictional forces make rotation difficult.Depending upon how much force is required, this could make intentionalrotation during implantation more difficult; particularly whenconsidering that the rotational force or torque applied is transferredalong a flexible member having the same length as lead body 16′. Anappropriate lubricant may be initially provided to ease rotation. Forexample, the friction lock may occur at the interface 121 between theprong support 105 and the fossa abrasive ring 64. A biocompatiblelubricant would be provided at the interface 121 and the lubricant wouldbreak down upon exposure to bodily fluids over an appropriate timeinterval.

A locking tab, detent or other mechanical member may be provided as ananti-rotation mechanism. That is, when the prong support rotation member110 is fully rotated the mechanical member is engaged and preventsreverse rotation. Thus, during implantation, the prong support rotationmember 110 is not fully rotated until proper placement is confirmed.FIGS. 5D and 5E illustrate one embodiment of utilizing a protruding tab82 in combination with a detent 80 to prevent reverse rotation. Certainelements of the assembly 50 are not illustrated for purposes of clarity.

FIG. 5D illustrates the prong support 105 and a plurality of prongs 100.A portion of an interior housing support 75 (see also FIG. 5B) is shownrelative to the prong support 75. As illustrated, the prong support 105is forward of the interior housing support 105. The protruding tab 82extends from a rear surface (as illustrated) of the prong support 105towards the interior housing support 75. The detent 80 is providedwithin the interior housing support 75 and is configured to receive theprotruding tab 82. FIG. 5D illustrates the prong support 105 prior tocomplete rotation in the direction of arrow 120. At this point, rotationin either direction is permitted. FIG. 5E illustrate protruding tab 82engaged within the detent 80. Rotation in either direction is nowprecluded.

The protruding tab 82 may be a fixed member that abuts the interiorhousing support 75 and the spring tension of the prong support 105causes the protruding tab 82 to engage the detent 80. Alternatively, theprotruding tab 82 may be a spring loaded member. Finally, the protrudingtab 82 may be releasable via a number of mechanisms. In one embodiment,the protruding tab 82 is mechanically retracted by a member operablefrom a distal end of lead 16′. In another embodiment, the protruding tab82 and detent 80 are shaped such (e.g., angled wall or walls) thatsufficient force may be applied to cause the protruding tab 82 to exitthe detent 80. Alternatively, the interior housing support 75 may bemoved in a proximal direction, thus separating the tab 82 from thedetent 80. It should be readily apparent that a number of alternativesexist that preclude unintentional reverse rotation while providing theoption to deliberately remove or reposition the device.

Referring to FIGS. 7A-7D, another embodiment of pressure sensor assembly50 is illustrated. In this embodiment, a plurality of piercing prongs300 are illustrated. Specifically, four prongs 300 a-300 d are disposedabout the interior housing support 75. The prongs 300 pierce through thefossa ovalis 36 and secure the housing 70. Each piercing prong 300 isdisposed within a prong track 310, with tracks 310 a and 310 b beingillustrated. The prong deployment mechanism 58 (linear, threaded orotherwise) is advanced in the direction of arrow F. This drives the head315 of the prong 300 through the tissue. More specifically, the head 315includes a piercing tip 320 and one or more locking tabs 355 that pivotwith respect to the main axis of the prong 300. An anchor recoil spring305 is provided for each prong 300; with recoil springs 305 a and 305 billustrated. Thus, deployment of the prong 300 must include sufficientforce to overcome the spring tension of the anchor recoil spring 305 andto pierce the relatively strong tissue of the fossa ovalis 36.

FIG. 7B schematically illustrates the prongs 300 prior to deployment.Anchor recoil springs 305 are either free of tension or as illustrated,retain the prongs 300 in a proximal position. In FIG. 7C, force has beenapplied in the direction of arrow F. The prongs 300 have pierced thefossa ovalis and the force applied by the prong deployment mechanism 58is reduced or eliminated. The anchor recoil springs 305 are compressedin the process and consequently, exert a force in a direction oppositethat indicated by arrow F. As such, the anchor recoil springs 305 exertthis force against the prongs 300, causing them to move in a proximaldirection. As the tabs 355 are pivotablly coupled to the prong 300, thismovement causes the tabs 355 to engage the tissue of the fossa ovalis 36surrounding the piercing point and open as shown. In this manner thetabs 355 are anchored against the wall of the fossa ovalis 36 within theleft atrium 40. As such, the pressure sensor 60 is maintained in theappropriate position. FIG. 7D illustrates the piercing heads 315 andanchoring tabs 355 for each of the prongs 300 a-300 d. Because thepiercing heads 315 and tabs 355 protrude minimally into the left atrium40, they will typically not disrupt fluid flow in such a manner as togenerate clotting. Furthermore, over time tissue growth will encapsulatethe piercing heads 315 and tabs 355 serving both to further anchor theassembly 70 and to obviate the presence of a foreign body in the leftatrium. It should be appreciated that the anchor prongs 52 of FIGS.4A-4D could be utilized in a similar manner. That is, rather thandeploying into the fossa limbus, the anchor prongs 52 could deploy intoand/or through the fossa ovalis 36 to secure the housing 70. Forexample, the anchor prong tract 55 (FIG. 4B) could continue linearlyrather than angling parallel to the fossa ovalis 36. Alternatively, thehousing 70 could be reconfigured such that a portion of the housing 70is in contact with the fossa limbus 38 so that the above describedanchor prong variation pierces the fossa limbus 38. The remainder of thehousing 70 would be configured so that contact is still maintainedbetween the sensor 60 and the fossa ovalis or as described below, thesensor 60 is advanced forward of the housing 70 to contact the fossaovalis 36.

FIGS. 8A-8C illustrate an embodiment of sensor assembly 50 similar tothat of FIGS. 7A-7D. In this embodiment, sensor capsule 60 may beadvanced and retracted towards and away from the fossa ovalis 36. Themovement of the sensor capsule 60 is applicable to any of the embodimentdescribed herein and is not limited with to the embodiment includingprongs 300 that pierce into the left atrium 40.

In summary, sensor capsule 60 is advanced and retracted via rotation,which engages a threaded member and translates rotation movement intolateral movement. In the illustrated embodiment, sensor capsule 60includes a threaded section 400 that engages a corresponding threadedtrack 410 disposed within the interior housing portion 75. A sensoractuation member 420 is connected to or coupleable with the sensorcapsule 60 and permits rotation of the sensor capsule 60 from theproximal end of lead 16′.

FIG. 8B illustrates the sensor capsule 60 in a retracted position and isnot in contact with the fossa ovalis 36. As such, a gap 430 is presentbetween the distal end of the pressure sensor capsule 60 and the fossaovalis 36. The housing 70 may be secured into position using the variousembodiments described while this gap 430 is maintained. Subsequently,the sensor actuation member 420 is rotated and the pressure sensorcapsule 60 is advanced toward the fossa ovalis 36. The amount of lineartravel may be selected based on several considerations. The pressuresensor capsule 60 should traverse the entire gap 430 such that at leastminimal contact is made with the tissue of the fossa ovalis 36. In otherwords, the capsule 60 should contact the right atrial wall 435. Furthermovement in this direction increases the tension between the sensorcapsule 60 and the tissue. Depending upon the particular sensor used,this may result in a better signal output. Continued advancement maycause the capsule 60 to enter the tissue of the fossa ovalis. Thepressure sensing capsule may be positioned such that its distal face isdisposed along any plane between the right atrial wall 435 and the leftatrial wall 445. The tissue thickness 456 is simply the thickness offossa ovalis 36 at this location and defines the maximum amount oflateral movement for the pressure sensing capsule 60 prior to entry intothe left atrium. Naturally, as force is applied this tissue thickness456 will be reduced in practice due to compression and deflection.

The above described maximum amount of lateral movement is defined byprecluding entry into the left atrium 40. The pressure sensor capsule 60could be caused to pierce through the fossa ovalis 36 and enter the leftatrium 40. In much the same manner as the piercing prong 300 minimallyprojects into the left atrium 40, advancement of the sensor capsule 60could be similarly limited. Thus, tissue encapsulation from new tissuegrowth would be the only attenuating factor for pressure sensing.

Though illustrated as traveling a relatively short span, the threadedtrack 410 could extend the entire length of lead 16′, allowing forcomplete separation of the sensor capsule 60 from the lead body 16. Thiscould be utilized during implantation; that is, the lead body 16′ andhousing 70 act as a catheter for the sensor capsule's deployment.Furthermore, it would facilitate replacement of the sensor capsule 60without necessitating replacement of the housing 70. It should also beappreciated that the threaded track need not extend the entire length oflead 16′. That is, the sensor capsule 60 could be advanced by a stylet(or the sensor actuation member 420 acting as a stylet) up to thethreaded track 410. Then, rotation of the capsule 60 will cause thethreaded section 400 to engage the threaded track 410. This permits themechanical advantage provided by the threaded engagement to take placeover the distance necessary to contact tissue without requiring thatmethod of travel over the entire length of the lead 16′.

FIG. 9 is a flowchart describing an overview of selecting and implantingthe above described pressure sensing assemblies. Initially, the fossaovalis of the patient is evaluated (500). This could involve imagingtechniques such as fluoroscopy, X-ray, CAT scan, MRI or the like;electrophysiological mapping, intracardiac echocardiography or any otherpatient specific technique. This is to discern the size and shape of thefossa ovalis so that an appropriate sensor assembly is selected 510. Asindicated, this may lead to a sensor assembly that is customized for agiven patient. Alternatively, a plurality of models are available andthe most appropriate of these models is selected. Finally, a singlestandard sensor assembly may be provided for all patients and if this isthe case or the selected option, the step of evaluating the fossa ovalis500 becomes optional.

Once the sensor assembly is selected, the “target” is defined (520) withrespect to the fossa ovalis. Typically, this means that the center pointof the fossa ovalis is identified. The housing assembly is thendelivered (530) to this target location. While multiple methods may beused, this typically includes the insertion of a catheter which isguided through the superior vena cava and into the right atrium. Thesensor housing is delivered through this catheter and positioned againstthe fossa ovalis at the targeted location. Using the various techniquesdiscussed above, the properly positioned housing is anchored (540) tothe fossa ovalis and/or the fossa limbus. This anchoring may include thegeneration of a vacuum as well as the advancement of one or moreanchoring prongs.

Finally, the pressure sensor itself is configured (550). This mayrequire that the pressure sensor move relative to the housing. Onceproperly positioned (either via this additional step or via the aboveanchoring procedure), movement of the fossa ovalis due to left atrialfluid pressure is measured by the pressure sensor and data is providedaccordingly. Over time, the effect of the implant, anchoring, andcontinued presence of a foreign body will cause the cardiac tissue toreact by generating tissue or fibrotic growth. This effect will tend toattenuate the output of the pressure signal as change occurs. Theprocess will eventually stabilize and the data provided by the pressuresensor will be relatively consistent.

As disclosed herein, a number of embodiments have been shown anddescribed. These embodiment are not meant to be limiting and manyvariations are contemplated within the spirit and scope of theinvention, as defined by the claim. Furthermore, particular elementsillustrated and described with respect to a given embodiment are notlimited to that embodiment and may be used in combination with orsubstituted into other embodiments.

1. An implantable medical device (IMD) comprising: a housing; a pressuresensor disposed within the housing and having a pressure sensinginterface disposed along a first plane; and a first deployable anchorprong deployable from a retracted position at least partially within thehousing to an extended position along a lateral path such that theanchor prong is operable to pierce laterally adjacent cardiac tissuewhen deployed to the extended position, wherein the anchor prong doesnot cross the first plane.
 2. The IMD of claim 1, further comprising: avacuum channel disposed within the housing; a vacuum grid disposed onthe housing and in a plane generally parallel to the first plane.
 3. TheIMD of claim 2, further comprising a check valve disposed within thehousing and in the vacuum channel such that when the check valve is in aclosed position a generated vacuum is maintained between the check valveand the vacuum grid when the vacuum grid is occluded.
 4. The IMD ofclaim 3, further comprising: a lead body coupled with the housing; alumen within the lead body coupled with the vacuum channel such thatevacuation of the lumen at a proximal end of the lead body generatesnegative pressure within the vacuum channel.
 5. The IMD of claim 1,further comprising a plurality of additional anchor prongs, eachdeployable from a retracted position to an extended position along apath, where none of the additional anchor prongs cross the first plane.6. The IMD of claim 5, wherein the first anchor prong and the additionalanchor prongs each include an electrode.
 7. The IMD of claim 5, whereinthe first anchor prong and the additional anchor prongs are eachindividually deployable.
 8. The IMD of claim 5, wherein the first anchorprong and the additional anchor prongs are collectively deployedtogether.
 9. The IMD of claim 1, wherein the first anchor prong includesa barb.
 10. The IMD of claim 9, wherein the barb is selectivelyretractable.
 11. The IMD of claim 1, further comprising: a pressuresensor mount disposed within the housing and supporting the pressuresensor, wherein the mount permits the pressure sensor to move in adirection generally perpendicular to the first plane.
 12. The IMD ofclaim 11, wherein the pressure sensor mount includes a threaded trackand the pressure sensor includes a threaded portion that engages thethreaded track such that rotation of the pressure sensor causes thepressure sensor to move in the direction generally perpendicular to thefirst plane.
 13. The IMD of claim 1, further comprising: an abrasivering coupled with the housing and disposed in a plane generally parallelto the first plane.
 14. The IMD of claim 13, wherein the abrasive ringincludes an uneven surface structure that facilitates tissue growth andencapsulation.